1. Field of the Invention
The present invention generally relates to an antenna radiating element and, more particularly, to a foursquare antenna element which can provide dual polarization useful in, for example, compact, wideband radar and communication antenna arrays.
2. Description of the Related Art
An antenna is a transducer between free space propagation and guided wave propagation of electromagnetic waves. During a transmission, the antenna concentrates radiated energy into a shaped directive beam which illuminates targets in a desired direction. In a radar system, the target is some physical object, the presence of which is to be determined. In a communication system, the target may be a receiving antenna.
During reception, the antenna collects energy from the free space propagation. In a radar system, this energy comprises a signal reflected back to the antenna from a target. Hence, in a radar system, a single antenna may be used to both transmit and receive signals. Likewise in a communication system an antenna may serve the dual functions of transmitting and receiving signals from a remote antenna. In a radar system, the primary purpose of the antenna is to determine the angular direction of the target. A highly directive, narrow beam-width is needed in order to accurately determine angular direction as well as to resolve multiple targets in physically close proximity to one another.
Phased array antenna systems are formed from an arrayed combination of multiple, individual, similar radiator elements. The phased array antenna characteristics are determined by the geometry and the relative positioning of the individual elements and the phase and amplitude of their excitation. The phased array antenna aperture is assembled from the individual radiating elements, such as, for example, dipoles or slots. By individually controlling the phase and amplitude of the elements very predictable radiation patterns and beam directions can be realized. The antenna aperture refers to the physical area projected on a plane perpendicular to the main beam direction. Briefly, there are several important parameters which govern antenna performance. These include the radiation pattern (including polarization), gain, and the antenna impedance.
The radiation pattern refers to the electromagnetic energy distribution in three-dimensional angular space. When normalized and plotted, it is referred to as the antenna radiation pattern. The direction of polarization of an antenna is defined as the direction of the electric field (E-field) vector. Typically, a radar antenna is linearly polarized, in either the horizontal or the vertical direction using earth as a reference. However, circular and elliptical polarizations are also common. In circular polarization, the E-field varies with time at any fixed observation point, tracing a circular locus once per RF (radio frequency) cycle in a fixed plane normal to the direction of propagation. Circular polarization is useful, for example, to detect aircraft targets in the rain. Similarly, elliptical polarization traces an elliptical locus once per RF cycle.
Gain comprises directive gain (referred to as "directivity" G.sub.D) and power gain (referred to as simply "gain" G) and relates to the ability of the antenna to concentrate energy in a narrow angular regions. Directive gain, or directivity, is defined as the maximum beam radiation intensity relative to the average intensity, usually given in units of watts per steradian. Directional gain may also be expressed as maximum radiated power density (i.e., watts/meter.sup.2) at a far field distance R relative to the average density at the same distance. Power gain, or simply gain, is defined as power accepted at by the antenna input port, rather than radiated power. The directivity gain and the power gain are related by the radiation efficiency factor of the antenna. For an ideal antenna, with a radiation efficiency factor of 1, the directional gain and the power gain are the same (i.e., G=G.sub.D).
Antenna input impedance is made up of the resistive and reactive components presented at the antenna feed. The resistive component is the result of antenna radiation and ohmic losses. The reactive component is the result of stored energy in the antenna. In broad band antennas it is desirable for the resistive component to be constant with frequency and have a moderate value (50 Ohms, for example). The magnitude of the reactive component should be small (ideally zero). For most antennas the reactive component is small over a limited frequency range.
Phased array antennas capable of scanning have been know for some time. However, phased array antennas have had a resurgence for modern applications with the introduction of electronically controlled phase shifters and switches. Electronic control allows aperture excitement to be modulated by controlling the phase of the individual elements to realize beams that are scanned electronically. General information on phased array antennas and scanning principles can be gleaned from Merrill Skolnik, Radar Handbook, second edition, McGraw-Hill, 1990, herein incorporated by reference. Phased array antennas lend themselves particularly well to radar and directional communication applications.
Since the impedance and radiation pattern of a radiator in an array are determined predominantly by the array geometry, the radiating element should be chosen to suit the feed system and the physical requirements of the antenna. The most commonly used radiators for phased arrays are dipoles, slots, open-ended waveguides (or small horns), and printed-circuit "patches". The element has to be small enough to fit in the array geometry, thereby limiting the element to an area of a little more than .lambda./4, where .lambda. is wavelength In addition, since the antenna operates by aggregating the contribution of each small radiator element at a distance, many radiators are required for the antenna to be effective. Hence, the radiating element should be inexpensive and reliable and have identical, predictable characteristics from unit to unit.
Radiator elements such as the "four arm sinuous log-periodic", described in U.S. Pat. No. 4,658,262 to DuHamel, and the Archaemedian spiral, which have wide bandwidths and are otherwise desirable for array applications have diameters greater than 0.43 .lambda. at their lowest frequency. With a bandwidth in excess of 1.5:1 in a square grid array an interelement spacing of about 0.33 .lambda. is desired.